Method and apparatus for generating hydrogen gas

ABSTRACT

A method of generating hydrogen gas is disclosed. The method includes a first step of contacting an aqueous solution of a chemical hydride and a catalyst and producing hydrogen gas and a heated hydrogen-depleted solution. The hydrogen gas is recovered and used as required, for example in a fuel cell. The heated solution is brought into direct or indirect heat exchange relationship with a metal hydride, thereby heating the metal hydride and causing desorption of hydrogen from the metal hydride and producing hydrogen gas and cooling the heated solution and producing a cooled solution. The hydrogen gas is recovered and used as required.

The present invention relates to method and apparatus for generatinghydrogen gas.

The present invention also relates to a fuel cell-based system forgenerating electricity.

A major disadvantage of power systems, such as fuel cells, that usehydrogen gas as a source of fuel is the difficulty in generating and/orstoring sufficient volumes of hydrogen gas in a safe and cost effectivemanner.

By way of particular example, this is a significant factor against theuse of hydrogen gas-powered fuel cells for motor vehicles.

The present invention is a method and apparatus for generating hydrogengas in a safe and cost effective manner.

Potentially, the method and apparatus of the present invention couldgenerate sufficiently large amounts of hydrogen gas so that hydrogen gasgeneration/storage is not a factor against the use of hydrogen-poweredfuel cells as an alternative to petrol-powered internal combustionengines.

According to the present invention there is provided a method ofgenerating hydrogen gas which includes the steps of:

-   (a) contacting an aqueous solution of a chemical hydride and a    catalyst and producing hydrogen gas and a heated hydrogen-depleted    solution;-   (b) recovering hydrogen gas produced in step (a);-   (c) bringing the heated solution produced in step (a) into direct or    indirect heat exchange relationship with a metal hydride and heating    the metal hydride and causing desorption of hydrogen from the metal    hydride and producing hydrogen gas and cooling the heated solution    and producing a cooled solution; and-   (d) recovering hydrogen gas produced in step (c).

The above-described method enables hydrogen gas to be recovered from twosources of hydrogen, namely chemical hydrides and metal hydrides, in asafe and an energy-efficient manner.

Specifically, the method is based on the realisation that the heatedhydrogen-depleted solution that is produced in chemical hydride reactionstep (a) can have sufficient thermal energy to heat metal hydrides to atemperature at which hydrogen is desorbed from the metal hydrides at anacceptable rate.

In situations where the cooled solution produced in step (c) isalkaline, preferably the method further includes contacting the solutionwith a metal and producing hydrogen gas and recovering the hydrogen gas.

A suitable metal is aluminium.

Preferably the method further includes treating the cooled solutionproduced in step (c) to regenerate the chemical hydride.

Preferably the method includes treating the cooled solution produced instep (c) to regenerate the chemical hydride by electrolysis of thecooled solution in an electrolytic cell that contains an ionic liquid,such as dicarb, as an electrolyte.

The electrolysis may be direct electrolysis of the cooled solution inthe electrolyte, with some hydrogen gas generation and with the hydrogengas being captured by the metal hydride.

The electrolysis may be indirect electrolysis with the cooled solutionand the electrolyte being in separate compartments of the electrolyticcell and being separated by a barrier that is selectively permeable toions that can form the chemical hydride, whereby the ions migrate fromthe compartment containing the cooled solution into the compartmentcontaining the ionic liquid in response to an applied potential. Thechemical hydride or a precursor of the chemical hydride may form as agas or as an insoluble compound in the ionic liquid. The chemicalhydride or precursor may then be extracted and recycled.

Preferably the barrier is impermeable to water and thereby preventsmigration of water from the compartment containing the cooled solutioninto the compartment containing the ionic liquid.

Preferably step (c) includes heating the metal hydride by at least 30°C.

More preferably step (c) includes heating the metal hydride by at least40° C.

It is preferred particularly that step (c) includes heating the metalhydride by at least 50° C.

The chemical hydride may be any suitable chemical hydride.

Suitable chemical hydrides include metal containing compounds such aslithium hydride, lithium aluminium hydride, and sodium borohydride andorganic hydrides such as dimethyl borane.

Sodium borohydride is a preferred chemical hydride.

Sodium borohydride is relatively stable but can produce large amounts ofhydrogen gas under suitable reaction conditions.

Specifically, sodium borohydride (NaBH₄) can react with water in thepresence of a suitable catalyst (such as ruthenium) and evolveconsiderable amounts of hydrogen gas in accordance with the followingexothermic reaction:NaBH₄+4H₂O→4H₂+NaOH+H₃BO₃

Sodium borohydride will not spontaneously react in water to evolvehydrogen gas and the catalyst is required to initiate the reaction.

As noted above, the reaction is exothermic. Consequently, thehydrogen-depleted solution resulting from the reaction is heated,typically by 50° C.

Sodium borate is one source of sodium borohydride.

The aqueous solution of the chemical hydride supplied to step (a) may bein the form of a slurry that includes a suspension of chemical hydrideparticles in water that contains chemical hydride in solution.

The metal hydride may be any suitable metal hydride.

Suitable metal hydrides include iron titanium hydride and lanthanumnickel hydride.

Iron titanium hydride is a preferred metal hydride.

The rate of desorption of hydrogen gas from metal hydrides istemperature dependent.

In the case of iron titanium hydride, significant amounts of hydrogengas can be evolved by heating the hydride by 30° C.

According to the present invention there is also provided a hydrogen gasgenerator that includes:

-   (a) a chemical hydride reactor for allowing contact between an    aqueous solution of a chemical hydride and a catalyst and producing    hydrogen gas and a heated hydrogen-depleted solution;-   (b) a metal hydride reactor for generating hydrogen gas by heating    metal hydride by direct or indirect heat exchange with heated    hydrogen-depleted solution from the chemical hydride reactor and    causing desorption of hydrogen from the metal hydride and generating    hydrogen gas and producing a cooled solution; and-   (c) a means for transferring heated solution from the chemical    hydride reactor to the metal hydride reactor.

According to the present invention there is also provided a system forgenerating electricity that includes a fuel cell and the above-describedhydrogen gas generator.

According to the present invention there is also provided anelectric-powered motor vehicle which includes the above-described systemfor generating electricity and a means for controlling the relativeamounts of hydrogen gas generated by the chemical hydride reactor andthe metal hydride reactor of the system.

The present invention is described further by way of example withreference to the accompanying flow sheet of a preferred embodiment ofthe present invention.

The preferred embodiment is described in the context of a motor vehiclethat is powered by electricity generated by a fuel cell that requireshydrogen as a feed material.

The present invention is not limited to this end-use application andextends to any other applications that require hydrogen.

With reference to the flow sheet, an aqueous slurry of a chemicalhydride, namely sodium borohydride, is supplied from a tank to a firstreactor and comes into contact with a catalyst, typically ruthenium, inthe reactor.

The contact between the sodium borohydride slurry and the catalystresults in the evolution of considerable amounts of hydrogen gas inaccordance with the following exothermic reaction:NaBH₄+4H₂O→4H₂+NaOH+H₃BO₃

The hydrogen gas is discharged from the first reactor and is supplied toa fuel cell to generate electricity.

In view of the exothermic reaction in the first reactor, the slurry isheated, typically by 50° C., in the reactor.

The hydrogen-depleted slurry discharged from the first reactor istransferred to a heat exchanger.

In the heat exchanger the hydrogen-depleted slurry is brought intoindirect heat exchange with a metal hydride, namely iron titaniumhydride, in a second reactor, with the result that the iron titaniumhydride is heated approximately 50° C.

Heating the iron titanium hydride causes desorption of hydrogen from theiron titanium hydride and produces hydrogen gas.

The hydrogen gas so-formed is discharged from the second reactor and issupplied to the fuel cell to generate electricity.

The indirect heat exchange between the hydrogen-depleted slurry from thefirst reactor and the iron titanium hydride cools the slurry by 20-30°C.

The cooled hydrogen-depleted slurry is transferred to a third reactorand is brought into contact with aluminium. The slurry is alkaline and,consequently, reacts with aluminium and generates hydrogen gas and heat.The hydrogen gas is discharged from the third reactor and is supplied tothe fuel cell to generate electricity. The heat is also supplied to thefuel cell and contributes to the thermal requirements of the fuel cell.

The cooled and now neutralised hydrogen-depleted slurry, which containsboron containing liquid, is transferred to a fourth reactor in whichsodium borohydride is regenerated.

Regeneration may take place in situ but more typically will occurthrough removal of the boron containing liquid and transfer to aseparate processing facility.

The regeneration may occur through the conventional processing route forsodium borohydride with conversion of the boron component into boricacid which then becomes feedstock for producing sodium borohydride forthe process. The conventional processing route involves reacting boricacid with methanol to produce tri-methyl borate which is then reactedwith sodium hydride at elevated temperatures. This yields sodiumborohydride and sodium hydroxide (caustic soda) together with methylproducts which can be re-used in the process as methanol, plus someimpurities and oils which are removed in a purification process.

More preferably, in a second regeneration option, regeneration will bethrough novel electrochemical processing involving the use of the newgeneration of electrolytes termed ionic liquids. These liquids have thecapability to allow electrolysis at relatively low temperatures atvoltages sufficient to drive the formation of strongly reducingcompounds such as sodium borohydride without dissociating themselves aswould be the case in an aqueous electrolyte.

The exact configuration will depend upon the specific ionic liquid. Insome cases the configuration will include direct electrolysis of theslurry, with some hydrogen gas generation and with the hydrogen gasbeing captured by the metal hydride system. In other configurations willinclude the removal of the water, possible separation of the aluminiumif present, and electrolysis of the sodium boron containing solution.

The direct electrolysis route has the attraction of in situ regenerationof the sodium borohydride during periods when the system is not requiredto produce hydrogen for feed to the fuel cell.

The ex situ use of ionic liquids may involve producing intermediatecompounds which are subsequently converted to a suitable hydride. In oneconfiguration the electrolysis would be carried out in an electrolyticcell which contains a membrane or diaphragm to separate the electrodecompartments, one of which contains an ionic liquid electrolyte. Thehydride species and/or a suitable intermediate is generated from theionic liquid containing compartment and separated out for use. Thisarrangement enables the hydride produced to avoid contact with waterfrom the solution being regenerated or originating from the electrolysisreaction and therefore minimises the chances of back reaction and lossof efficiency in the cell. This can then enable less stableintermediates which are highly reactive with water, such as diboranegas, to be generated during the electrolysis and captured for subsequentreaction.

A third regeneration option is to direct high temperature formation ofthe sodim borohydride from the boron-sodium liquor through hightemperature reaction in the presence of a hydrogen source, preferablynatural gas (=methane), and a strong reductant to drive the reaction.The reductant will preferably be one or a combination of relativelyinexpensive metals such as aluminium, sodium magnesium, silicon ortitanium and carbon possibly supplemented by some hydrogen gas.

In the context of an electric-powered motor vehicle in which electricityis generated by a fuel cell, the use of a chemical hydride (such assodium borohydride) and a metal hydride (such as iron titanium hydride)as sources of hydrogen gas, and the linking of the chemical hydride andmetal hydride reactions by using the heat generated by the chemicalhydride reaction to cause the metal hydride reaction, is a considerableadvantage.

The above-described system alleviates the volume efficiency and weightefficiency problems generally associated with the use of hydrogen inmotor vehicles. Specifically, the above-described system takes advantageof the volume efficiency of metal hydrides and the weight efficiency ofchemical hydrides. Moreover, the combination of metal hydrides andchemical hydrides compensates for the poor weight efficiency of metalhydrides and the poor volume efficiency of chemical hydrides.

In the context of motor vehicles, both chemical and metal hydrides arerenewable sources of energy. It is envisaged that motor vehicles bedesigned so that suitable “tanks” of sodium borohydride slurry (or othersuitable chemical hydride slurry) and iron titanium hydride (or othersuitable metal hydride) are replaced as required and the used sodiumborohydride and iron titanium hydride are regenerated for subsequentre-use.

In the context of a motor vehicle, the system further includes a controlmeans for regulating the supply of hydrogen generated in the firstreactor by catalytic reaction of sodium borohydride slurry and in thesecond reactor by desorption from iron titanium hydride.

Specifically, it is envisaged that the sodium borohydride be used togenerate hydrogen on ignition and during acceleration and that the irontitanium hydride be used to generate hydrogen during the other phases ofoperation of a motor vehicle.

Many modifications may be made to the preferred embodiment of thepresent invention described above without departing from the spirit andscope of the invention.

By way of example, whilst the preferred embodiment is described in thecontext of sodium borohydride and iron titanium hydride, the presentinvention is not so limited and extends to any suitable chemicalhydrides and metal hydrides.

By way of further example, whilst the preferred embodiment describes theuse of ruthenium as the catalyst, the present invention is not solimited and extends to any suitable catalyst for chemical hydrides.

1. A method of generating hydrogen gas which includes the steps of: (a)contacting an aqueous solution of a chemical hydride and a catalyst andproducing hydrogen gas and a heated hydrogen-depleted solution; (b)recovering hydrogen gas produced in step (a); (c) bringing the heatedsolution produced in step (a) into direct or indirect heat exchangerelationship with a metal hydride and heating the metal hydride andcausing desorption of hydrogen from the metal hydride and producinghydrogen gas and cooling the heated solution and producing a cooledsolution; and (d) recovering hydrogen gas produced in step (c).
 2. Themethod defined in claim 1 further including contacting the cooledsolution produced in step (c) with a metal and producing hydrogen gasand recovering the hydrogen gas in situations in which the cooledsolution is alkaline.
 3. The method defined in claim 2 wherein the metalis aluminium.
 4. The method defined in claim 1 further includingtreating the cooled solution produced in step (c) to regenerate thechemical hydride.
 5. The method defined in claim 1 further includingtreating the cooled solution produced in step (c) to regenerate thechemical hydride by electrolysis of the cooled solution in anelectrolytic cell that contains an ionic liquid as an electrolyte. 6.The method defined in claim 5 wherein electrolysis is indirectelectrolysis with the cooled solution and the electrolyte being inseparate compartments of the electrolytic cell and being separated by abarrier that is selectively permeable to ions that can form the chemicalhydride, whereby the ions migrate from the compartment containing thecooled solution into the compartment containing the ionic liquid inresponse to an applied potential.
 7. The method defined in claim 6wherein the chemical hydride or a precursor of the chemical hydride formas a gas or as an insoluble compound in the ionic liquid.
 8. The methoddefined in claim 7 further includes extracting the chemical hydride orprecursor from the ionic liquid.
 9. The method defined in claim 1wherein step (c) includes heating the metal hydride by at least 30° C.10. The method defined in claim 9 wherein step (c) includes heating themetal hydride by at least 40° C.
 11. The method defined in claim 10wherein step (c) includes heating the metal hydride by at least 50° C.12. The method defined in claim 1 wherein the chemical hydride includesany one or more of lithium hydride, lithium aluminium hydride, sodiumborohydride, and dimethyl borane.
 13. The method defined in claim 1wherein the chemical hydride is sodium borohydride.
 14. The methoddefined in claim 1 wherein the aqueous solution of the chemical hydridesupplied to step (a) is in the form of a slurry that includes asuspension of chemical hydride particles in water that contains thechemical hydride in solution.
 15. The method defined in claim 1 whereinthe metal hydride includes any one or more of iron titanium hydride andlanthanum nickel hydride.
 16. A hydrogen gas generator comprising: (a) achemical hydride reactor for allowing contact between an aqueoussolution of a chemical hydride and a catalyst and producing hydrogen gasand a heated hydrogen-depleted solution; (b) a metal hydride reactor forgenerating hydrogen gas by heating metal hydride by direct or indirectheat exchange with heated hydrogen-depleted solution from the chemicalhydride reactor and causing desorption of hydrogen from the metalhydride and generating hydrogen gas and producing a cooled solution; and(c) a means for transferring heated solution from the chemical hydridereactor to the metal hydride reactor.
 17. The generator defined in claim16 further comprising means for regenerating the chemical hydride fromthe cooled solution produced in the metal hydride reactor.
 18. Thegenerator defined in claim 17 wherein the means for regenerating thechemical hydride includes an electrolytic cell.
 19. The generatordefined in claim 18 wherein the electrolytic cell includes twocompartments separated by a barrier that is selectively permeable toions that can form the chemical hydride.
 20. A system for generatingelectricity comprising a fuel cell and a hydrogen gas generatorcomprising: (a) a chemical hydride reactor for allowing contact betweenan aqueous solution of a chemical hydride and a catalyst and producinghydrogen gas and a heated hydrogen-depleted solution; (b) a metalhydride reactor for generating hydrogen gas by heating metal hydride bydirect or indirect heat exchange with heated hydrogen-depleted solutionfrom the chemical hydride reactor and causing desorption of hydrogenfrom the metal hydride and generating hydrogen gas and producing acooled solution; and (c) a means for transferring heated solution fromthe chemical hydride reactor to the metal hydride reactor.
 21. Anelectric-powered motor vehicle which comprising a system for generatingelectricity, said electricity generating system comprising a fuel celland a hydrogen gas generator, wherein said hydrogen gas generatorcomprises: (a) a chemical hydride reactor for allowing contact betweenan aqueous solution of a chemical hydride and a catalyst and producinghydrogen gas and a heated hydrogen-depleted solution; (b) a metalhydride reactor for generating hydrogen gas by heating metal hydride bydirect or indirect heat exchange with heated hydrogen-depleted solutionfrom the chemical hydride reactor and causing desorption of hydrogenfrom the metal hydride and generating hydrogen gas and producing acooled solution; and (c) a means for transferring heated solution fromthe chemical hydride reactor to the metal hydride reactor and a meansfor controlling the relative amounts of hydrogen gas generated by thechemical hydride reactor and the metal hydride reactor of said system.